Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close()

From: Peter Hurley
Date: Wed Jul 09 2014 - 09:58:00 EST

On 06/17/2014 07:32 AM, Peter Hurley wrote:
On 06/17/2014 07:03 AM, David Laight wrote:
From: Peter Hurley
I don't understand the second half of the changelog, it doesn't seem
to fit here: there deadlock that we are trying to avoid here happens
when the *same* tty needs the lock to complete the function that
sends the pending data. I don't think we do still do that any more,
but it doesn't seem related to the tty lock being system-wide or not.

The tty lock is not used in the i/o path; it's purpose is to
mutually exclude state changes in open(), close() and hangup().

The commit that added this [1] comments that _other_ ttys may wait
for this tty to complete, and comments in the code note that this
function should be removed when the system-wide tty mutex was removed
(which happened with the commit noted in the changelog).

What happens if another process tries to do a non-blocking open
while you are sleeping in close waiting for output to drain?

Hopefully this returns before that data has drained.

Good point.

tty_open() should be trylocking both mutexes anyway in O_NONBLOCK.

Further, the tty lock should not be nested within the tty_mutex lock
in a reopen, regardless of O_NONBLOCK.

AFAICT, the tty_mutex in the reopen scenario is only protecting the
tty count bump of the linked tty (if the tty is a pty).

I think with some refactoring and returning with a tty reference held
from both tty_open_current_tty() and tty_driver_lookup_tty(), the tty
lock in tty_open() can be attempted without nesting in the tty_mutex.

Regardless, I'll be splitting this series and I'll be sure to cc
you all when I resubmit these changes (after testing).

Peter Hurley

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